The present disclosure, according to certain embodiments, generally relates to particle separation. More specifically, the present disclosure, according to certain embodiments, relates to methods for separating magnetic nanoparticles.
The removal of particles from solution with magnetic fields may be, among other things, more selective and efficient (and often much faster) than traditional centrifugation or filtration techniques. As a result, magnetic separations may be used in fields including, but not limited to, biotechnology and ore refinement. In many cases, the process utilizes the generation of magnetic forces on particles large enough to overcome opposing forces such as Brownian motion, viscous drag, and sedimentation. In biotechnology, for example, magnetic separators may use relatively low field gradients in a batch mode to concentrate surface-engineered magnetic beads from a suspension. For manufacturing, magnetic materials may be recovered from waste streams under flow conditions with high-gradient magnetic separators (HGMS) that use larger fields (up to 2 Tesla) and columns filled with ferromagnetic materials.
Decreasing the particle sizes used in magnetic separations from microns to nanometers may increase the available sorptive areas by at least about hundred times. Such material optimization, however, is not generally practical because the magnetic force acting on a particle in a field gradient is proportion to the particle volume. If particles are too small, their magnetic tractive forces in a field gradient may not be large enough to overcome Brownian motion, and little or no separation may occur. For iron oxide, extrapolations from the behavior of the bulk material suggest that the critical size for separation is about 50 nm for the case of an isolated (non-aggregated) particle. This treatment assumes very large applied fields and the latest designs for extremely high-gradient separators, both features that may make magnetic separations prohibitively expensive in many settings. For simpler and less costly low gradient separators, the critical size for capture in magnetic gradients may increase substantially.
Extrapolations from bulk properties to nanoscale materials are frequently problematic, and a more comprehensive analysis of nanoscale magnetic behavior suggests that nanocrystals (NCs) could offer a significant opportunity for low field magnetic separations. Below about 50 nm diameter, nanoscale magnets may exhibit a complex range of size-dependent behaviors, including, but not limited to, a transition below about 40 nm in size to single domain character, magnetic susceptibilities greater than that of the bulk material, and the emergence of superparamagnetic behavior. Such systems may experience larger magnetic forces than expected from bulk behavior due to larger moments. Advantages of higher susceptibility materials, such as FeCo, have been suggested, in which an increased magnetic moment could in principle enable high-gradient separations with isolated nanocrystals. Additionally, in external fields, the large surface gradients present at the surfaces of single domain materials may induce transient aggregation, effectively forming larger and more magnetically responsive particles. Nanoparticle aggregation, even before field application, has been posited to explain the observation of the magnetic capture of polydisperse nanocrystals in a high-gradient separation (>1000 T/m) using fields of 1 to 2 Tesla.